As described in Patent Document 1, a hydraulic system for a work machine such as a hydraulic excavator generally includes a hydraulic pump, a hydraulic actuator driven by hydraulic fluid delivered from the hydraulic pump, a flow/direction control valve of the center bypass type for controlling the flow of the hydraulic fluid supplied from the hydraulic pump to the hydraulic actuator, operation means provided corresponding to each hydraulic actuator to operate each flow/direction control valve, and a pump regulator that controls the displacement of the hydraulic pump in such a manner that the delivery amount of the hydraulic pump changes according to the operation on the operation means.
Some of the hydraulic systems for work machines such as hydraulic excavators include, for various purposes, a center bypass cut valve arranged at the downstream end of a center bypass line extending through the flow/direction control valve of the center bypass type. Patent Document 1 describes an example of such a hydraulic system. Specifically, when the operation means is operated so as to supply the hydraulic fluid to a cylinder chamber on the load holding side, the center bypass cut valve is operated in a direction for reducing the center bypass opening area, and the delivery pressure of the hydraulic pump is controlled to be higher than the load pressure of the cylinder. Accordingly, deterioration in the fuel efficiency is prevented through a reduction in the energy loss and excellent low-speed operability is achieved.
In recent years, there have been proposed work machines improving their energy efficiency by driving the upper swing structure by use of a hydraulic motor and an electric motor, thereby making the electric motor function as a generator at times of deceleration or stoppage of the upper swing structure, and recovering energy in the form of electric power.
For example, in a work machine described in Patent Document 2, the center bypass cut valve is formed by use of a reserve spool which normally exists in the control valve unit. When the swing electric motor is set in a non-driving state, the reserve spool is controlled so as to throttle the open center hydraulic line by use of a composite restrictor formed of a bleed-off restrictor of the flow/direction control valve and a bleed-off restrictor of the reserve spool, by which control is performed to raise the meter-in pressure of the swing hydraulic motor and thereby increase the drive torque of the swing hydraulic motor. With this control, excellent operability and work performance are secured even when the need of stopping the function of the electric motor arose due to a failure in the electric motor, a voltage abnormality in the electrical storage device, or the like.